1. Field of the Invention
The present invention relates to the testing of electric characteristics of integrated circuits formed on a semiconductor wafer. More specifically, the present invention relates to a wafer probing test apparatus having a test head that is docked with a probe card though which electrical signals are passed to and from integrated circuits on a semiconductor wafer.
2. Description of the Related Art
Generally, integrated circuits are fabricated on a wafer and are then electrically tested before being packaged to ensure the reliability of the circuits. Electrical die sorting (EDS) is used for electrically testing the unpackaged integrated circuits as they are formed on the wafer. One test device for use in carrying out EDS is a wafer probing machine. The wafer probing machine tests the integrated circuits by placing probe needles in direct contact with metallic pads formed on the integrated circuits.
The conventional wafer probing machine comprises a test head which can move linearly up and down, and a pogo block having a plurality of pogo pins installed on the bottom of the test head. A probe card is installed in a lower part of the overall apparatus, and a wafer chuck is provided under the probe card for supporting the semiconductor wafer to be tested. The test head is docked with the probe card during the testing process.
The probe card is usually replaced on occasion, for example when the type of semiconductor wafer being tested changes or when the probe card is damaged or broken. Whenever the probe card is replaced, the wafer probing machine must be reset. Every time the wafer probing machine is reset, the test head must be precisely docked once again with the probe card.
More specifically, the test head of the wafer probing machine must be brought to a predetermined position with respect to the probe card during the resetting of the wafer probing machine. In this case, the test head is raised to a certain height and then is forced downwardly to dock with the probe card. At this time, the test head should be precisely positioned relative to the probe card, with respect to both its polarity and vertical alignment. Otherwise, the test head will collide with the probe card, resulting in damage to the probe card or poor contact between the pogo pins of the test head and probe needles of the probe card. If either of these conditions occurs, the results of the testing of the integrated circuits become unreliable.
However, it is very difficult to determine whether the test head is properly positioned relative to the probe card. Also, the position at which the test head is docked with the probe card is typically adjusted manually by a test operator. Therefore, the docking position is never precise. In fact, tests shown in FIG. 6 reveal that the docking position may vary in height by several mms throughout the course of resetting the wafer probing machine several times.
Furthermore, if the operator forces the test head and the probe card together during the docking thereof, the probe card tends to be bent downward at the center portion thereof. In this case, probe needles of the probe card become inclined toward the center of the probe card. As a result, the contact between the probe needles and the pads formed on the semiconductor device (the contact height) becomes uneven. Furthermore, the ends of the probe needles are worn down when the operator forces the test head to dock with the probe card while the docking position is not precise.
In addition, the probe card may adhere to the test head as if by suction. In this case, the probe card tends to be bent upwardly at the center portion thereof when the docking position is not precise. As a result, the probe needles are deformed. Therefore differences occur in the contact heights between the probe needles and the pads during a hot test, i.e., a test performed at a high temperature.
An improved technology for securing the probe card to the test head is disclosed in Japanese Patent Laid Open Publication No. Hei 11-30647. Referring to this publication, the test head has guide pins, and corresponding guide holes are formed in the probe card. The probe card is received in a head plate and is secured thereto by a floating clamp ring. The docking of the test head with the probe card is thus facilitated by the guide pins and guide holes when the test head is moved downwardly towards the probe card. Furthermore, the shock caused by the downward force of the test head is absorbed by an elastic member interposed between the clamp ring and the head plate.
However, this technology still presents some problems. For example, it is difficult to detect whether the test head is properly docked with the probe card because the test operator evaluates the docking state by eye by trying to observe the state of engagement of the guide pins and guide holes. Furthermore, it is still difficult to prevent the test head from striking the probe card with an excessive force because the operator operates the test head manually.